This invention relates to a power control switch of the type employing a solid state switching element.
Power switches for DC applications commonly use pulse width modulation. In its simplest form this comprises the power switch being driven at a particular frequency, with the switching element switched to an "on" position for a certain proportion of each cycle, the duty cycle, in which the energy transmitted depends on the proportion of the cycle for which the switching element is in the "on" position. Pulse width modulation is commonly achieved by using a comparator to compare a sawtooth waveform with a reference voltage and applying the comparator output via buffering to the base of a bipolar transistor. The duty cycle can be increased or decreased by varying the reference voltage such that the proportion of each cycle for which the reference voltage is exceeded is correspondingly increased or decreased.
In certain power switching applications it is important that the switch be efficient for one of two reasons. The first reason is that if the switch is inefficient then considerable heat is generated within the switch, which reduces the power handling capability of the switching element and/or requires heat sinking for the switching element and consequent heat exchanging apparatus, which can substantially dictate the size of the switch. Secondly, in some DC applications the power is supplied by a finite source, namely a battery, and therefore it is particularly important that the energy stored in that battery is used efficiently, especially in such applications as electric vehicles where the battery size itself is limited. Therefore there is a requirement for a highly efficient power control switch.
The recent development of the insulated gate bipolar transistor (IGBT) overcomes the inefficiency associated with the large drive current required for conventional bipolar transistors. However, another power loss associated with bipolar transistors, both conventional and IGBTs, in pulse width modulated applications, is that which arises due to the relatively slow turn-off time of typically 500 ns or more. During this period the voltage across the switching element increases while current continues to pass, especially if there is a high inductance in the circuit, as there would be if the load was, for example, an electric motor. The power dissipated in the transistor is the product of its voltage and current. It will be readily appreciated that the efficiency of the bipolar transistor will decrease as the frequency increases due to the increase in the number of switching transitions. The problem is therefore compounded by the desirability in many applications to drive the power control switch at a frequency outside the range of human hearing, i.e. above 20 kHz.
One solid state switching device which has a considerably faster turn-off time than the IGBT is a metal oxide semiconductor field effect transistor (MOSFET), which has a turn-off time of approximately 20 ns. However, compared with the IGBT, it has a considerable resistance while in its "on" state. In addition high current handling MOSFETs are expensive and require high volume heat sinks. To overcome the problems associated with the relatively slow switching time of the IGBT, it has been proposed in an article by Matthew Carter, published in PCIM Europe May/June 1991 pages 150-156, to connect in parallel an IGBT and MOSFET (shown in FIG. 15 of the article). The MOSFET is controlled such that it conducts for approximately 300 ns after the IGBT turns off such that there is no voltage across the IGBT during the period when the free electron density is decaying. In this arrangement the IGBT handles the conduction losses which are minimized by the IGBT having a low resistance, whereas the MOSFET handles switching losses which are minimized by the MOSFET having a relatively short switching time of approximately 20 ns. This combination of IGBT and MOSFET is also disclosed in European Patent Application No. 0502715.